The British Airways flight BA94 left Montréal–Pierre Elliott Trudeau International Airport at 9.30pm EDT and is due to land at 8.50am BST.
It is not yet clear what the emergency involves.
The plane is flying over the UK and will come to land soon.
BA94 was flying at an altitude of 41,000ft above the Atlantic when the crew alerted Heathrow to an onboard situation.
The emergency was classed as 'general'.
British Airways flight BA94 is coming in to land at Heathrow while declaring an emergency
The incident comes one day after an Air Canada plan landed in London without a wheel.
On Tuesday the tyre snapped off mid-air, shortly after take-off on the Montreal to Heathrow flight.
The missing wheel was found in a fence just a few miles from Pierre Elliott Trudeau Airport in Montreal.
After landing damage to the Boeing 777-300's suspension was also reported.
Heathrow Airport: A plane coming in from Canada is declaring an emergencyA spokeswoman for British Airways said the emergency was called after a passenger was taken ill.
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She said: “The safety of our customers and crew is always our priority. “Our pilots have requested a priority landing at Heathrow because someone has been taken ill on board. “The aircraft has already landed.”